Well, we bombed the Moon, so I don't know why anyone would be surprised that we'd nuke the ocean too. Although I imagine it was a rather unpleasant experience for the passengers and crew of the ship that got swallowed up by the massive tidal wave that ensued in the aftermath of the atomic explosion.

Why? Seems like the government likes to talk about environmental awareness a lot for doing stuff like this. "Hmm...what needs more pollution....the ocean! Yes, the ocean needs more radioactivity! We will make super fish-people!"

They matched the audio to the video so that the sound was in time with the bomb when in reality there would have been a delay due to the speed of sound. Because the audio finishes 18 seconds early and the speed of sound is 343.2m/s, that tells us the camera was positioned approximately 6.2 milometers or 3.9 miles away from the site of the explosion, assuming that the end of the video corresponds with the end of the audio track in time.

Depending on the test - goats were put in various locations on and inside the vessels to determine survivability from close-in nuclear explosions. Check wikipedia's article on 'Operation Crossroads' under the section headed 'Radiation'